Integration: Lighting and HVAC systems
By considering the first principles of radiative energy, engineers can determine how to balance daylight, electric light, and HVAC systems, particularly considering various options for daylight control and the advent of LEDs.
- Understand key principles that affect the integration of lighting and HVAC systems.
- Understand the effects of and design strategies for daylighting with regard to lighting and HVAC considerations.
- Understand the ways in which electric lighting impacts interior HVAC internal heat gains,and the impacts of LED lighting.
What is the most fundamental connection between lighting and HVAC? Daylight and the sun both impact lighting and HVAC. Both require the support of electrical systems for power. But the physics of radiation could be considered the most fundamental link between lighting and HVAC,and of intrinsic importance for building integration possibilities.
Most building occupants probably would link lighting (vision) and sound (hearing), rather than making an obvious connection between lighting and HVAC-where temperature and humidity would be linked to our skin’s sense of touch. Most people do not easily connect lighting and HVAC, partly because temperature and humidity are more intuitive concepts to us than are sound and light, which seem more abstract.
It’s when balancing or coordinating both the lighting and thermal environments of a building design that the importance of radiative energy transfer is more obvious. Both the lighting designer and the mechanical engineer for any building project are accounting for radiative energy transfer in their designs, but the importance of the synergies between the two are not always apparent.
With visible light, radiation is essentially the only method of energy transfer in light generation,reflection, absorption, and our perception. Visible light and infrared radiation operate on relatively similar scales of wavelength and frequency compared to sound or other radiated energy categories. It’s somewhat incidental that our eyes perceive wavelengths near 550 nanometers as visible light, but not ultraviolet (UV) wavelengths shorter than 400 nm or infrared wavelengths longer than 700 nm. But in considering design integration and synergies of lighting and HVAC,radiation at multiple wavelengths must be a paramount consideration. Thermodynamics and the thermal environment involve more than just radiative considerations; they also involve convection and conduction. However, those have very little to do with lighting in basic principle and are outside the scope of this article.
The sun’s radiation is clearly important to us on Earth-providing the light that we see arriving on the ground, and the heat that we sense on our skin (as well as UV radiation, which we recognize when we’re exposed for too long). So whatever you call it-natural light, daylight, or solar heat gain-solar radiation is a clear priority for both lighting and HVAC design.
Also, electrically energized light sources (or artificial light sources, electric light sources, or simply"lights") emit light by radiative transfer, and have historically emitted most of their waste heat by radiative transfer in the same manner and direction as their light output.
Before electric lighting or control systems, daylight was the chief integration opportunity for the design process and a successful outcome of HVAC and lighting. Consider that daylight includes directional sunlight, diffused sky and cloud illumination, and at the same time the visible, infrared,and UV components from these sun and sky sources. For the best potential results, daylighting must be considered very early in the design process for any building.
The direct sun primarily impacts glare and thermal comfort, while the diffuse sky primarily impacts useful natural illumination and some additional infrared radiation. The windows and glazing systems that are so important to occupant enjoyment, and provide daylight inside to offset electric light source use, also admit solar heat into buildings. With more attention paid to well-insulated and airtight building envelopes, the heat of the sun radiating onto a building has a greater chance to accumulate and overheat a building-or to cause notable loads that the building cooling system must offset, with resulting energy use.
So maximum possible daylight is not a win-win solution. Daylight within the limits of useful interior quantities is a partial "win," allowing us to turn off or dim electric lights and reduce some internal heat gains from those lights. But every hour of the year that indoor daylight exceeds occupant lighting needs, there is a risk for solar heat gain greater than needed, and HVAC energy spent in cooling the building more than needed.
So how do we maximize the daylight for the sake of electric light savings (and occupant appreciation, happiness, enhanced productivity, sleep-wake cycles, property value, merchandise sales rate, etc.), while also minimizing the problematic side effects? In addition to solar heat gain associated with daylight and glazing, there’s a fine line between allowing appropriate amounts of daylight and minimizing hours that occupants close blinds for glare. At first, one may begin to think that shading from glare is probably simultaneously shading from excess heat gain, but this is not necessarily the case.
For glare sources, people are most sensitive to brightness directly in the center of their line of sight, as indicated by the use of the Guth position index in many glare analysis metrics (such as CIE 117). Additionally, research ongoing since 2006 related to a relatively new metric called daylight glare probability shows that the illuminance received at one’s eye particularly affects a person’s perception of glare. And the light sources most significantly impacting illuminance atone’s eye are those sources central to one’s line of sight. So for glare, the sun at the horizon is often the most problematic.
Considering solar heat gain instead of glare, the sun at the horizon is typically not a problem during morning and evening hours. Correlating solar heat gain to external temperature, the most important times to shade a building from the sun are most likely the times that outdoor temperature is high-also the same midday hours that the sun is high in the sky. People are much less sensitive to glare from sources high above their line of sight. Also, sun high in the sky is less likely to reach more than a few workstations that might be intentionally close to the façade.
So the higher the sun gets in the sky, the more important it is to shade the building from solar heat gain. The lower the sun is in the sky, the more important it is to shade the windows from potential glare-to give a chance that window blinds stay open and useful light can get through.
Shading and siting
Is there any time that we don’t need to shade and protect a building from the solar environment?
Go back to the earlier statement that "for the best potential results, daylighting must be considered very early in the design process for any building." The best way to allow daylight with the fewest chances of problematic sun side-effects is to create the best shape of the building and the best location for windows.
Windows facing east or west will get glare from the sun at low angles in the sky every day(morning sunrise on the east, morning sunset on the west), and they also will have the high angle and high heat sun at midday for at least part of the time before or after noon.
Windows facing south (or facing north in the Southern Hemisphere) will get the high-angle, high-heat sun throughout most of the day all year, but will not have the sunset or sunrise directly ahead of them. In favorable locations, this can simplify shading requirements for the high angle sun, and alleviate the need for shading of lower angle sun positions. In locations farther from the equator, however, the low-angle sun in winter could cause glare at more times of day. Because winter sunrise and sunset are more southerly, they have a notable impact on the southern exposure.
Windows facing north (or facing south in the Southern Hemisphere) are the ideal for avoiding solar heat gain while admitting useful diffused daylight. When the sun rises or sets in the northerly sky during the summer, it’s typically very early or very late in the day and outside of normal work hours, with no need for glare protection. When the sun is high in the sky and the outside temperatures are high, the sun is never on the side of the building that faces away from the earth’s equator.
If every building was constructed in an anecdotal "vacuum," then every building could be a long floor plate, with circulation spaces tolerant of temperature variations along the south façade, firestairs on the east and west ends of the building with no need for daylight, and a long façade of windows for occupied spaces facing away from the equator. These theoretical buildings would also need to have all the beautiful landscape features and distant city skyline views in the same direction that the occupied spaces are getting their diffuse daylight from-if we want to avoid disrupting this idealized scenario.
In the real world, buildings are limited to particular sites, with street frontage and size of lots impacting building shape; desirable views to natural or manmade landmarks in various directions(or avoiding undesirable views); and resulting in windows facing east, south, west, and north. Still,early design considerations prevail over engineering solutions. For example, can cafes, break rooms, lounges, lobbies, and other transient spaces with easier acceptance of glare and temperature variation be located where the sun is most problematic? Can the spaces that require the least light-that can have the smallest windows and use the least energy when blinds are closed-be located to the east and west where blinds will be closed daily at sunrise and sunset?
After we assume the building has at least partially non-ideal orientation of occupied spaces, how can we accommodate daylight in a way that considers the best outcome for both lighting and thermal concerns? It becomes necessary to be selective about what portions of the sun’s radiation are admitted to the building. A building can select what wavelengths of sun to admit, at what times sun is admitted to each window, and what angles of sun to admit.
Glazing and shading
Selecting wavelengths could be considered a part of the status quo approach. It is typical that in climates requiring multi-pane insulated glazing, the glazing is likely to have a low-emissivity coating. In milder climates, a film or interlayer with particular reflectance and transmittance properties may take the place of low-e coating to limit infrared transmission into a building, while still allowing proportionally more visible light indoors. Even when additional solar design techniques are employed, the benefits of coatings to tailor wavelength transmission can be valuable. Filtering wavelengths of light can improve the HVAC situation by limiting sun, but it can’t improve the issues of glare and requirements for window blinds.
Selecting times of day also could be considered part of the status quo in many cases, although there is quite a range of methods by which solar impact on buildings can be filtered temporally-running the gamut from user-operated interior window blinds to computer-controlled and motor-operated exterior louver systems. The fallback lowest risk option is manually operated window blinds: people can always lower the blinds when needed, the cost and complexity of installation is basic, and if the shades are left down by occupants, the mechanical system doesn’t suffer,though electric lighting stays energized to compensate and counteracts much of the intention for the glass in the first place.
If interior window blinds can be motorized and automated, it is a marginal improvement that the blinds are opened at least once a day, and admit useful visible radiation (daylight) for most of the day until a person or an automated sensor closes the blinds-at the times that blinds must be down to prevent glare for workplace comfort and productivity.
When simple blinds are internal, they might reflect a bit of infrared energy back through the glass outward to the exterior, but some of the heat has already been absorbed into the fabric or venetian blinds and transmitted through them to the building interior. If an automated motorized shading system can be moved to the building exterior, it can block as much infrared heat from the building at some times as it can block the visible glare at other times, and serve a more useful dual purpose. A more advanced and complex step is to combine automation for various times of day and seasons, with angular solar selectivity.
Selecting angles of radiation to admit can be done even without automation, by using fixed louvers, cellular panelized modules, overhangs, and so on. In traditional architecture, this could take the form of window shutters and jalousie systems, eaves and overhangs, colonnades and porticos, mashrabiya, and more. While architectural styles evolve over time, and these traditional shading forms may be reserved for historical revivals, the current necessity for cautious use of energy brings notably visible shading systems back to the forefront of architecture. It is only with notable shading systems that any degree of expansive glazing design can be responsibly enjoyed.
The most productive angular selection tends to be a horizontally biased louver or shading grid to cut out the highest and hottest sun angles. On the east and west, one often reiterated but problematic thought might be to implement vertical louver-like systems. The low-angle sun can probably only be shaded for limited times of the year by such a system, while a horizontal shading system can block some mid-day sun throughout the year. However, if an angularly selective set of louvers can be adjustable and motorized, they might be able to adjust at various times of day to block both the high-angle sun for heat concerns, and the low-angle sun for glare concerns at other times, while rarely blocking as much daylight as a non-angular shading system would block.
Aside from selective shading of solar radiation, non-selective general shading such as fritting and mesh shade screens is another option. While blanket-coverage shading on the outside of a façade can reduce solar heat gain, it likely reduces potentially useful visible light to the same degree, and limits this useful light at all hours, including when there is no glare or heat gain concern. In some cases, such as frit or diffusing interlayers, these approaches may even reduce visible radiation more so than infrared, which may help glare or excess illumination exposure but may not have as notable a double-duty benefit as infrared protection at the same time.
There is one more key consideration in optimal and smart design and planning of glazing: striving toward glass of the size and specification for useful coverage and transmission without excess.While admitting radiation in the form of useful light and infrared heat gain, glass is also commonly a chink in the armor of the building envelope’s insulation. So excess glass might allow excess heat into the building during the day beyond useful illumination and views, while also allowing excess heat to leave the building at night and possibly adding to heating energy expenditure. Glazing panels are always limited in their insulating ability compared to the potential insulation of other portions of the building envelope, making excess glass at least a three-fold problem in glare, infrared heat, and insulation impacts.
Once the planning of glass and daylight for a building has been considered-with the balance of solar gains, useful light, and insulation requirements-the electric lighting fixtures and their visible and infrared radiation should be considered. What are the key factors in coordinating or integrating HVAC and internal electric lighting systems? It is important to acknowledge the changing technology of electric lighting and its impacts on the way its heat is radiated, to judge the future potential for the next generation of buildings.
In original incandescent lighting, metallic (tungsten) filaments within light bulbs are heated by electricity running through them, and creating heat and light due to the resistivity of those filaments. The typical glass bulb-shaped enclosures trap heated air around the filament, while infrared (and some UV) radiation is largely emitted in the same directions as the illumination. With the advent of halogen lighting, the trapped air temperature is used with halogen gas fill within the bulb, to power a cycle by which small traces of evaporated filament are deposited back to the filament for extended life and efficiency. However, the waste heat is still primarily radiated to the same places of the building as the visible light. Only a tiny portion of waste heat is conducted back through a standard light socket, and some heat convected away from the glass bulb of the incandescent or halogen lamp.
Fluorescent and discharge lamps represent an entire generation (or multiple generations) of lighting technology based on running electricity through element-filled chambers to produce a glowing arc, rather than a glowing hot filament. Whether using low-pressure mercury in a large fluorescent tube with phosphorescent coatings, or high-pressure mercury, metal halides, and/or sodium in a tiny chamber within a bulb, the electrical arcs through these elements radiate more visible and UV wavelengths and less infrared waste than incandescent lamps. The electrodes used in such products still conduct electricity and generate heat through their resistance, but with the glowing arc of gaseous metals replacing filaments as the primary light source, electrodes can be more robust and long-lasting, instead of thin enough to radiate notable light output themselves.
So the gases and phosphors of fluorescent and other discharge lamps have been more efficient at creating more light and less heat, compared to incandescent. These arc lamp technologies also require ballast components to modulate the electrical current to strike and maintain the glowing arc between electrodes. So some waste heat is moved to a ballast. At the lamp itself,with more of the electrical energy converted to light (and UV), less infrared radiation is generated directly, and more of the waste heat is convected away from the lamp and ballast.
Now with LEDs, the norm is changing again. Generating light through electrical excitation and interaction of electrons across semiconductor diodes, most LEDs used for illumination emital most no infrared or UV radiation. They are still not 100% efficient, however, and so waste heat is generated directly within the diode (and electronic components), which must be conducted and convected away.
Critically, LED semiconductors are extremely sensitive to heat and temperature. Just as computer components would overheat without fans or heat-sink strategies, LED light output will fade severely and electrical components could fail if not protected from the heat they generate. This is a new concern for lighting, as incandescence is practically insensitive to temperature, and fluorescent lamps are more sensitive to cold temperatures impacting gas pressures within bulbs.Ballasts may have been sensitive to extreme high temperatures, but only with LEDs do we have a source that can cook itself to the point of failure even in room temperature conditioned space.
Similar to computers, LEDs are typically laid out on circuit boards of some form, whether small or large. Various strategies are taken for these circuit boards, but in all cases, the circuit board must conduct away sufficient heat generated by the diodes to prevent overheating. In low-power linear or planar LED fixtures with diodes distributed along larger circuit boards, the circuit board itself may be substantial enough to conduct and dissipate sufficient heat from dispersed low-power diodes. In LED systems with power and intensity concentrated to a small intense light source, the circuit board must be assisted with a heat-sink to further conduct heat away, and to better dissipate this heat to the surroundings through increased surface area, or even through fan-like systems and airflow design. Also, instead of a ballast controlling electrical arc, a combination of driver and transformer (also known as power supply) operate LEDs, and generate a portion of the heat from the system.
Keep in mind however, that although LEDs are not directly radiating infrared heat into the room,the visible radiation is still eventually being absorbed into the room surfaces (after some inter-reflection and useful illumination). LEDs are decreasing internal heat gains from lighting within buildings only by the small margin that they can be more efficient than whatever they are replacing (currently a small but beneficial gain compared to linear fluorescent, and quickly improving). Whether it is convected from a heat-sink or radiated from a filament, all electrical energy used in a lighting system becomes internal energy gain to be accounted for in building HVAC design.
But we are now in an age of LED lighting systems that essentially radiate only visible light into the direction of intended usefulness. Heat is concentrated at the source of the light, and these LED light sources will be very happy (with increased lifespan and efficiency) if the heat is dissipated quickly to keep them as cool as possible. We are still in an era of learning to design with, modularize, standardize, and maintain this new LED lighting technology, and at the same time it is evolving to be more efficient and more cost-effective with less waste heat. Opportunities to ingrain heat mitigation for the benefit of LED into the next paradigm of building systems design are ripe for implementation.
With appropriate management of solar radiation (and without the electrical energy expenditure of manmade lighting), daylight can be tailored to direct useful illumination where and when it is required, while mitigating solar gains through spectral, angular, or temporal selectivity-just as the thermal characteristics of LED systems make heat a critical risk to be mitigated, but with potential to more carefully accommodate or embrace these characteristics through building design.
With these principles in mind, the designer’s challenge is to determine the best combination of design implementations that holistically address the needs of both lighting and HVAC for buildings that fulfill occupant needs, bring people joy or make life comfortable, and avoid or at least limit detrimental impacts to the Earth. Some open-source tools that can help to assess and quantify the benefits of various design solutions include software like OpenStudio, Ladybug and Honeybee modeling software plug-ins, and all the other work behind the scenes of these software packages such as Radiance, EnergyPlus, or Daysim.
Chris Rush is senior lighting consultant at Arup, responsible for design, consultancy, and management for lighting projects throughout the U.S. and internationally, including all aspects of electric lighting, daylight, and lighting controls. He focuses on detailed coordination of daylight analysis and electric light and control designs with Arup’s whole building energy models, HVAC design loads, and building automation systems.